FIG. 1 shows schematically a cross-section through a multiple flame-tube combustor of a gas turbine engine. A number of flame-tubes 200 (only one shown in FIG. 1) are spaced circumferentially around the combustor, each flame-tube having a radial airspray burner 202. The flame-tubes are without interconnectors, and thus each has two igniter plugs 204 (only one shown in FIG. 1) and a locating pin fuel atomiser 206 used for starting only. Air admission port tubes 208 are welded to the barrel of the flame-tube. The flame-tubes 200 have thermal barrier coatings (TBCs) on their inner (hot) sides. The flame-tubes are single-skinned and cooled by cooling air films produced by angled effusion holes.
A double-walled casing system surrounds the flame-tubes. The outer 210 and inner 212 walls of the system form between them an outer volume 211 which gathers compressed air received from the high-pressure compressor of the engine for onward delivery to an exhaust gas recuperator. The inner wall 212 and the flame-tubes 200 form between them an inner volume 213 which receives the returning compressed air from the recuperator for delivery into the flame-tubes.
Each igniter plug 204 enters its flame-tube 200 through an aperture in the wall of the tube, with a substantially annular igniter boss 214 welded to the tube at the aperture to encircle the tip of the plug. The tip is thereby located such that it can provide an electrical discharge to ignite an air-fuel mixture within the flame-tube 200, the boss providing a mounting position for a seal for the igniter plug. The boss can also provide air holes which supply scrubbing air jets to the tip.
For example, as shown schematically in FIG. 2, each igniter boss 214 has an inner surface 215 which is flush with the hot side of the wall of the flame-tube 200. The boss also has a number of 3.0 mm diameter holes 216 at an angle of about 17.5° to the face 217 of the igniter tip and the hot side of the wall. Air from the inner volume 213 is directed by the holes 216 into air jets 218, the trajectory of which just misses the end of the bore of the boss 214 on the opposite side to the exits of the holes. The low angle of the air jets results in them scrubbing the tip of the igniter, such that they remove large fuel droplets that can reach the tip during ignition. The low angle air jets also combine with an upstream cooling air film 220 created by circumferential angled effusion holes to create an enhanced downstream cooling air film 222 across the hot side of the wall of the flame-tube.
When the combustor is used with corrosive fuels, such as marine diesel fuels, the inner surface 215 of the igniter boss 214 can quickly become corroded.
One option is to coat the inner surface 215 with a suitable resistant coating. However, this can increase costs as igniter bosses are in any event susceptible to wear due to the proximity of the igniter electrical discharge and thus in general have to be replaceable items.
Another option, shown schematically in FIG. 3, is to retract the igniter boss 214 and the igniter plug 204 slightly from the flame-tube 200 such that the wall of the flame-tube can be extended to cover over the inner surface 215 of the boss. This can protect the boss from corrosive fuels. However, the air jets 218 from the holes 216 then impact on the edge of the flame-tube wall defining the aperture rather than combining with the cooling air film 220 to create the enhanced downstream cooling air film 222. As a result, hot spots can be formed downstream of the boss, resulting in TBC loss, metal cracking and unacceptable combustor life. Further, the disturbed airflow around the igniter tip 217 can inhibit the drying of the tip face 217 and increase the amount of fuel droplets being transported into the bore of the boss 214. These fuel droplets can coke and cause short circuit malfunctions within the igniter assembly, resulting in an unacceptable igniter plug life and compromising start reliability.
If the holes 216 are adjusted so that the angle of the air jets 218 is increased, the air jets can be prevented from impacting on the edge of the flame-tube wall. However, the increased air jet angle causes less effective scrubbing of the igniter tip face 217. In addition, the increased angle hinders the air jets from combing with the cooling air film 220.
An object of the present invention is to provide an igniter assembly which addresses such problems.